Engine accessory drives systems and methods

ABSTRACT

An accessory drive for an engine includes a power takeoff (PTO) configured to couple power from a rotating shaft of the engine and to convey the power through an opening in a housing of the engine. A gearbox is coupled to and configured to be driven by the PTO. The gearbox is disposed external to the housing and includes a planetary gear train. At least one engine accessory is coupled to and configured to be driven by the planetary gear train.

BACKGROUND

1. Technical Field:

This disclosure relates to gas turbine engines in general, and in particular, to an accessory drive for such engines.

2. Related Art:

Engines, such as jet turbine engines of a type used, for example, to propel commercial or military aircraft, require a number of engine accessories, such as electrical generators, oil and fuel pumps, hydraulic and coolant pumps and the like, in order to function. To operate these essential accessories, some of the power generated by the engine is taken off of the engine and coupled through an “accessory drive,” or “gearbox,” that is used to drive the accessories. The accessories, along with the gearbox, are typically coupled closely to the outer surface of the engine's housing, and within the engine's nacelle, if any, to minimize the space they occupy, e.g., for streamlining purposes.

Engines typically include one or more rotating shafts from which “secondary power” can be coupled to drive the engine's accessories through the gearbox. However, existing engine accessory gearboxes, which typically incorporate one or more trains of spur gears, entail a fixed relative speed between the engine and the accessories without the possibility of stopping or changing the relative speeds or loads of the accessories. This can result in gear backlash between the engine and the accessories when they are respectively sped up, slowed down, stopped, or operated intermittently, with concomitant shaft vibration and torsional resonance.

SUMMARY

In accordance with this disclosure, one or more novel embodiments of engine accessory drives are provided for example that may substantially confine backlash in the drives to backlash between the shaft(s) of the engines and their respective gearboxes, and that may substantially eliminate backlash between the engine shaft(s) and each accessory. As an example, the accessories may not be affected by shaft vibration and torsional resonance and no or very little power may be lost due to rotating accessories that are not producing any output power.

In one example embodiment, an accessory drive for an engine comprises a power takeoff (PTO) configured to couple power from a rotating shaft of the engine and to convey the power through an opening in a housing of the engine. A gearbox is coupled to and configured to be driven by the PTO. The gearbox is disposed external to the housing and includes a planetary gear train. At least one engine accessory is coupled to and configured to be driven by the planetary gear train.

In another example embodiment, a method for driving at least one accessory of an engine comprises receiving power from a rotating shaft of the engine and conveying it through an opening of a housing of the engine with a power takeoff (PTO), driving a gearbox with the PTO, the gearbox being disposed external to the housing and including a planetary gear train, and driving the at least one engine accessory with the planetary gear train.

In yet another example embodiment, a method for driving a plurality of engine accessories, each of which is operable at a different speed and/or load than the speed and/or load of the other engine accessories, comprises driving the accessories with a planetary gearbox such that gear backlash between the engine and the accessories is substantially confined to backlash between the engine and the gearbox and substantially eliminated between the gearbox and the engine accessories.

The scope of the invention is defined by the appended claims, which are incorporated here by reference. A better understanding of the novel accessory drives of the present disclosure, as well as an appreciation of the above and additional advantages thereof, will be afforded to those of some skill in the art by a consideration of the following detailed description of one or more example embodiments thereof. In this description, reference is made to the various views of the appended sheets of drawings, which are briefly described below, and within which like reference numerals are used to identify like ones of the elements illustrated therein.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a stylized left side perspective view of a gas turbine engine incorporating an example embodiment of an engine accessory drive in accordance with the present disclosure, showing a plurality of engine accessories being driven by the accessory drive;

FIG. 2 is a stylized front end perspective view of the example engine accessory drive of FIG. 1;

FIGS. 3A and 3B are stylized front and side elevation views, respectively, of the accessory drive and engine accessories of FIG. 1;

FIG. 4 is a stylized side elevation view of an alternative embodiment of an engine accessory drive incorporating a differential-type of planetary gear train; and

FIG. 5 is a partial cutaway perspective view of a differential-type planetary gear train.

DETAILED DESCRIPTION

An engine, such as an aircraft gas turbine engine, typically includes a compressor, a combustor a turbine, and one or more rotating shafts that connect the turbine to the compressor and couple power between the two. The rotating shafting can include, for example, a low pressure (LP) shaft and a high pressure (HP) shaft, and in some embodiments, the two shafts can be coupled together to comprise a common shaft. The present disclosure contemplates the use of two separate engine “spools,” viz., an HP or compressor shaft having a plurality of compressor blades affixed to and extending radially outward from it, and an LP or turbine spool, i.e., a turbine shaft having a plurality of turbine blades affixed to and extending radially outward from it. Each of the two spools rotates within a corresponding one of the compressor, which comprises a chamber incorporating a plurality of fixed compressor blades extending radially inward within it, and the turbine, which comprises a chamber incorporating a plurality of fixed turbine blades extending radially inward within it.

In a typical axial-flow gas turbine engine for an aircraft, the engine typically supplies power for various systems and accessories essential to the engine and/or aircraft operation, as well as passenger comfort, besides producing a propulsive thrust that can include a combination of a “bypass” air stream and a turbojet exhaust thrust. One way to power these systems and accessories is to couple power from the LP shaft, the HP shaft, or a common shaft, typically located along a central longitudinal axis of the engine, with a transfer, or power takeoff (PTO) shaft. This shaft, which can comprise a “tower shaft” and/or an “angle shaft,” transmits the power to a gearbox, which, in some embodiments, can be mounted beneath the engine. In particular, the tower shaft can engage either the HP shaft or the LP shaft through a bevel gear set and transfer power from the shaft to the gearbox through another bevel gear set disposed adjacent to or within the gearbox.

The engine accessories can include, for example, at least one of a starter, an electrical generator, a fuel pump, a hydraulic pump, a scavenger pump, a de-oiler, or the like. As contemplated herein, the accessories can be driven at the same or substantially different speeds by means of a planetary gear train that is driven by a common transfer or PTO shaft. As also contemplated in the present disclosure, the planetary gearbox can include more gearing and/or additional shafts other than those discussed herein. Additionally, the gears themselves can comprise a wide variety of gears types, such as spur, crown, helical, hypoid, spiral gears, or the like.

In some embodiments, a brake and clutch can be used to selectively couple the PTO shaft to the LP shaft, the HP shaft, or the common shaft, and in a preferred embodiment, to the LP shaft for constant rotation. The gearbox, which can be located between the front and rear ends of the engine, transfers power from the selected engine shaft and also functions as a speed reduction unit.

The power taken off of the engine's LP, HP or common shaft (i.e., the secondary power) is used to power the accessories, which are coupled to the shaft through the gearbox (e.g. electric generator, hydraulic pumps, starters, and the like), and which can entail power, weight, maintenance, and safety demands, and can constitute a significant fuel penalty on the engine. The magnitude of these effects depends on the amount of power extracted, the operating conditions of the engine, the type of power extracted, and the point of power extraction within the engine.

Existing engine accessory gearboxes have a fixed relative speed between the accessories without the possibility of stopping or changing relative speeds of the accessories without producing backlash in the gears and attendant shaft vibration and torsional residue. However, by using a gearbox with a planetary gear arrangement, the gear backlash is substantially confined to backlash between the engine's LP, HP or common shaft and the planetary gearbox, and substantially reduced between the accessories themselves and/or the gearbox, so that relatively tight coupling of the accessories can aid in the control of shaft vibration and torsional residue.

For example, the fixed torsional mode frequency between accessories due to the constant relative speed of a traditional gearbox, and that of the non-constant relative speed of the accessories of the current disclosure, provides an advantageous opportunity to change the resonant mode frequency between the accessories of the present disclosure to other frequencies through a selective control method. For example, controlling the displacement of a variable displacement hydraulic pump by controlling the position of its swash plate. Similar control methods can be used with other engine accessories. Thus, the engine accessories of the present disclosure can start, stop, speed up or down, and/or operate at a continuous speed, all at the same time, while controlling the torsional mode frequencies of the accessories. This can result in a gas turbine engine with improved efficiency and reduced secondary power usage, which in turn, can result in a decrease in the fuel penalty due to the extraction of the secondary power from the engine shaft(s).

FIG. 1 is a stylized left side perspective view of a gas turbine engine 10 incorporating an example embodiment of an engine accessory drive 100 in accordance with the present disclosure, showing a plurality of engine accessories 102, 104 and 106 being driven by the accessory drive 100, and FIG. 2 is a stylized front end perspective view of the example accessory drive 100 of FIG. 1. FIGS. 3A and 3B are stylized front and side elevation views, respectively, of the accessory drive 100 and engine accessories 102, 104 and 106 of FIG. 1.

As discussed above, the engine 10 can comprise, for example, a turbojet engine or turbofan engine, and as illustrated in FIG. 1-FIG. 3B, the example engine 10 can comprise an upstream compressor section 12 coupled to a downstream turbine section 14, and a combustion chamber 16 disposed between the compressor and turbine sections 12 and 16. As discussed above, the rotating shafting of the engine 10 can comprise a low pressure (LP) shaft and a high pressure (HP) shaft, and in the particular embodiment illustrated, comprises an axially extending common shaft 18 (shown as a dashed line in FIG. 1) that couples the compressor section 12 to the turbine section 14 through the combustion chamber 16.

In the particular example embodiment illustrated, the engine accessories comprise a compressor 102, a pump 104, and an electrical motor/generator 106, but as discussed above, the type and number of the accessories driven by the accessory drive 100 can vary, depending on the particular application at hand. For example, the motor/generator 106 could alternatively or additionally comprise an engine starter, and the pump could alternatively or additionally comprise one or more of a fuel pump, a hydraulic pump, and/or an oil pump.

As illustrated in FIG. 1-FIG. 3B, the example engine accessory drive 100 comprises a power takeoff (PTO) 118 that is configured to couple power, i.e., secondary power, from the rotating common shaft 18 of the engine 10, and to convey the power through, for example, an opening 20 in a housing 22 of the engine 10, and a gearbox 120 that is coupled to and configured to be driven by the PTO 118. In the embodiment shown, the gearbox 120 is disposed external to a housing 22 of the engine 10, and of importance, includes a planetary (also known as an epicyclical or hypocycloidal) gear train 122, described in more detail below. As also described in more detail below, the engine accessories 102, 104 and 106 are coupled to and configured to be driven by the planetary gear train 122.

In the particular example embodiment of FIG. 1-FIG. 3B, the planetary gear train 122 comprises a sun gear 124, three planet gears 126 disposed on a planet carrier 128 and in driving engagement with the sun gear 124, and a ring gear 130 disposed in driving engagement with the planet gears 126, and is configured such that the planetary gears 126 are fixed against rotation with respect to the planet carrier 128, such that rotation of the sun gear 124, which is driven by a drive shaft 132 of the PTO 118, causes conjoint rotation of the planet gears 126, the planet carrier 128 and the ring gear 130, that is, the sun gear 124, the planet carrier 128 and the ring gear 130 all rotate at the same direction and at the same speed.

The engine accessory 102 is driven by the sun gear 124 through a drive shaft 134 that is coupled directly to and disposed coaxial with the shaft 132 that drives the sun gear 124. The engine accessory 104 is coupled to and driven by a shaft 136 having a gear 138 at its input end that is disposed in engagement with the ring gear 130, and the engine accessory 106 is coupled to and driven by a shaft 140 having a gear 142 at its input end that is disposed in engagement with the planet carrier 142. Of course, other gearing and coupling arrangements in the planetary gear drive 122 are possible and easily implemented. For example, the planet gears 126 can be allowed to rotate relative to the planet carrier 126 and revolve about the sun gear 124, the ring gear 130 can be fixed, and the engine accessories 104 and 106 can be driven by the planet carrier 126. Alternatively, the planet carrier 126 can be fixed, the ring gear 130 allowed to rotate, and the engine accessories 104 and 106 can be driven by the ring gear 130.

As illustrated in FIGS. 2 and 3B, the PTO 118 can comprise a so-called “tower shaft” 144 having an input end with, e.g., a first bevel gear disposed thereon, and an output end having a second bevel gear 146 disposed thereon. The bevel gear at the input end of the tower shaft 144 can, in turn, be coupled to and driven by another bevel gear disposed inside the housing 22 and concentrically upon the common shaft 18 of the engine 10. Thus, by varying the angle of these two “internal” bevel gears, the tower shaft 144 can be made to intersect the rotating common shaft 18 of the engine 10 at an angle that can varied from about 0 to about 90 degrees, that is, in a direction that can be varied between a direction substantially perpendicular to the common axis 18 of the engine 10, to a direction substantially parallel to the common axis 18.

As illustrated in FIGS. 2 and 3B, in some embodiments, the PTO 118 can further comprise an “angle shaft” 148 having another bevel gear 150 disposed on an input end thereof and in driving engagement with the bevel gear 146 disposed on the output end of the tower shaft 144, and between an output end of the tower shaft 144 and an input end of the planetary gear train 122. Further, as illustrated in, e.g., FIG. 2, another bevel gear 152 on the output end of the angle shaft 148 can be disposed in driving engagement with a bevel gear 154 disposed on the input end of the sun gear 124 drive shaft 132. As above, the respective angles of the bevel gears 146, 150, 152 and 154 can facilitate a wide range of mounting positions between the tower shaft 144 and the angle drive 148, and hence, between the engine 10 and the planetary gear accessory drive 100.

As discussed above and illustrated in FIG. 2, a brake 156 and/or a clutch 158 can be disposed between the tower shaft 144 and the common shaft 18 of the engine 10 and used to selectively couple or decouple the secondary power from the common shaft 18 and/or to slow or stop the planetary gear train 122.

FIG. 4 is a stylized side elevation view of an alternative embodiment of an engine accessory drive 160 incorporating a differential-type of planetary gear train 162, and FIG. 5 is a partial cutaway perspective view of the differential-type planetary gear train 162. The planetary gear train 162 illustrated in FIGS. 4 and 5 is similar in nature to the planetary gear train 122 of FIGS. 2 and 3B, but different in several respects, including that the latter uses bevel gears throughout, incorporates two sun gears 124, and is limited to driving only two engine accessories, e.g., accessories 104 and 106, as illustrated in FIG. 4.

As illustrated in FIGS. 4 and 5, in the differential planetary gear train 162, the bevel gear 146 at the output end of the tower shaft 144 drives the bevel gear ring gear 130 rotationally. The planet carrier 128 is fixed to the ring gear 130 for conjoint rotation therewith. The two bevel planet gears 126 are coupled to the planet carrier 128 for both rotation about their about their respective centers relative planetary carrier 128, and for conjoint rotation with the planetary carrier 128. The two bevel sun gears 124 are disposed in spaced opposition to each other and in engagement with both of the planet gears 126. Each of the sun gears 124 is respectively coupled to one of the engine accessories 104 and 106 through a corresponding one of their drive shafts 136 and 140.

The differential planetary gear drive 126 has, among others, the following properties. An input torque applied to the ring gear 130 by the tower shaft 144 causes both the ring gear 130 and the planet carrier 128 to rotate conjointly, thereby causing the two planet gears 126 to rotate conjointly with the planet carrier 128, without rotating about their respective centers. Since the planet gears 126 are both disposed in engagement with the two sun gears 124, this causes both sun gears 124, and hence, both engine accessories 104 and 106, to rotate in the same direction and at the same speed as the ring gear 130, the planet carrier 128 and the planet gears 124.

However, if one of the engine accessories, e.g., engine accessory 106, encounters resistance caused by, e.g., an increase in the load of the engine accessory 106, the planet gears 126 will be caused to rotate about their respective centers, as well as rotating conjointly with the planet carrier 128. This enables the engine accessory 106, together with its corresponding driving sun gear 124, to slow down, and results in a corresponding increase in the rotational velocity of the engine accessory 104, together with its corresponding driving sun gear 124, thereby enabling the two accessories 104 and 106 to operate at different rotational speeds.

As discussed above, in some embodiments, the engine accessories 102, 104 and 106 can be configured to operate in advantageous cooperation with related system components. For example, as illustrated in FIG. 2, the compressor 102, which operates at the speed of the sun gear 124, can be used in combination with a pair of closed circuits comprising a wing deicing circuit, in which a fluid heated by compression in the compressor 102, is circulated through an aircraft wing 164 to deice it, and then through a heat exchanger 166, before being returned to the compressor 102. In a second circuit, the passage of the fluid through the heat exchanger 166 functions to cool a lubricating fluid disposed in a sump 168 of the accessory drive 100 and circulated through the heat exchanger 166 by a pump 170 before being returned to the sump 168.

In another example, the pump 104 of FIG. 2 can comprise a hydraulic pump operating at the speed of the ring gear 130 and in conjunction with an accumulator 172 disposed in parallel with the pump 104, and a control valve 174 disposed at the outlet of the pump. When the pump 104 has filled the accumulator 172, e.g., at maximum displacement, the valve 174 at the outlet of the pump 104 can be shut off. Closing the valve 174 causes a maximum torque to be applied to the pump 104, and hence, to the motor/generator 106, described in more detail below. When this condition has been achieved, the pump 104 will stop rotating. When the pump 104 stops rotating, no power will be lost to friction at the pump 104, and a stopped pump can enable engine 10 starting using the generator/motor 106 at a maximum torque and gear ratio.

In yet another example, the motor/generator 106, which is driven at the speed of the planet carrier 128, can be used in conjunction with a battery 176 disposed in parallel with the motor/generator 106. In a manner similar to that discussed above in conjunction with the hydraulic pump 104 and hydraulic accumulator 172, the electric load on the engine 10 can be made more “discrete” (i.e., on or off) using the battery 176 in lieu of or in addition to the motor/generator 106.

Thus, the engine accessory drives of the present disclosure can eliminate the necessity of a fixed relative speed between the accessories by implementing a planetary gear train in the accessory drive gear box, thereby decreasing fuel penalties due to the unnecessary extraction of power from a shaft of the engine. By confining the gear backlash to backlash between the engine shaft and the planetary gearbox and not between each accessory, the accessories are relatively unaffected by shaft vibration and torsional resonance, enabling a relatively tight coupling of the accessories to the engine and reducing platform subsystem power off-take by reducing lost power due to rotating accessories when no accessory output power is required.

The planetary gear train of the accessory drive also enables variable speed of the engine accessories relative to the engine PTO shaft(s). The accessories can all start and stop, either together or independently of each other. One engine accessory, such as a refrigeration unit, can be operated steadily, while the speed of another accessory, such as an electrical power unit, is varied. The compressor of a refrigerator unit can be run at continuous speed to provide a continuous load. A hydraulic accumulator can be charged at optimal power using the engine shaft power, and an associated hydraulic pump can be stopped simultaneously, using a shutoff valve, or simultaneously varied using a swash plate of the pump. Thus, all system hydraulic demands can be served by the accumulator when the hydraulic pump is stopped, and no power will be lost due to unnecessary shaft motion.

A variable electric generator can also be sped up or down advantageously using an associated battery. The operation of the other accessories coupled to the present accessory drive can also be made more efficient by stopping their rotation when no accessory output power is required, thereby reducing lost power due to unnecessary rotation of the accessories.

Indeed, as those of some skill will by now appreciate, and depending on the specific application at hand, many modifications, substitutions and variations can be made in and to the materials, apparatus, configurations and methods of use of the planetary gear train accessory drives of present disclosure without departing from its spirit and scope. In light of this, the scope of the present invention should not be seen as limited to those of the particular embodiments illustrated and described herein, as they are merely by way of some examples thereof, but rather, should be fully commensurate with that of the claims appended hereafter and their functional equivalents. 

What is claimed is:
 1. An accessory drive for an engine, the accessory drive comprising: a power takeoff (PTO) configured to couple power from a rotating shaft of the engine and to convey the power through an opening in a housing of the engine; a gearbox coupled to and configured to be driven by the PTO, the gearbox disposed external to the housing and including a planetary gear train; and at least one engine accessory coupled to and configured to be driven by the planetary gear train.
 2. The accessory drive of claim 1, wherein the engine comprises a turbofan engine.
 3. The accessory drive of claim 1, wherein the rotating shaft of the engine comprises a low pressure (LP) shaft or a high pressure (HP) shaft of the engine.
 4. The accessory drive of claim 1, wherein the PTO comprises a tower shaft.
 5. The accessory drive of claim 4, wherein the tower shaft comprises an input end having at least one of a bevel gear, a crown gear, or a spur gear disposed thereon.
 6. The accessory drive of claim 4, wherein the tower shaft intersects the rotating shaft of the engine at an angle of from about 0 to about 90 degrees.
 7. The accessory drive of claim 4, further comprising an angle shaft disposed between an output end of the tower shaft and an input end of the planetary gear train.
 8. The accessory drive of claim 1, further comprising a clutch and a brake disposed between the PTO and the rotating shaft.
 9. The accessory drive of claim 1, wherein the planetary gear train comprises a sun gear, a planet gear, a planet carrier, and a ring gear, and the at least one engine accessory is coupled to and driven by the sun gear, the planet gear, the planet carrier, or the ring gear.
 10. The accessory drive of claim 1, wherein the planetary gear train comprises a differential, and the at least one engine accessory is coupled to and configured to be driven by a first sun gear or a second sun gear of the differential.
 11. The accessory drive of claim 10, wherein the differential comprises a bevel gear differential, and wherein the at least one engine accessory comprises an electrical generator, a pump, a compressor, or an engine starter.
 12. The accessory drive of claim 11, wherein the pump comprises a fuel pump, a hydraulic pump, or an oil pump.
 13. A method for operating the accessory drive of claim 1, the method comprising varying a rotational speed and/or a load of at least one engine accessory during operation of the engine.
 14. The method of claim 13, wherein the accessory comprises a variable displacement pump, and wherein the varying comprises varying a position of a swash plate of the pump.
 15. A method for driving at least one accessory of an engine, the method comprising: receiving power from a rotating shaft of the engine and conveying it through an opening of a housing of the engine with a power takeoff (PTO); driving a gearbox with the PTO, the gearbox disposed external to the housing and including a planetary gear train; and driving the at least one engine accessory with the planetary gear train.
 16. The method of claim 15, wherein the receiving of the power comprises: driving a second gear on an input end of a first shaft with a first gear on the rotating engine shaft; and extending an output end of the first shaft through the opening in the engine housing.
 17. The method of claim 15, wherein the driving of the gearbox comprises driving a fourth gear on an input end of the planetary gear train with a third gear on the output end of the first shaft.
 18. The method of claim 17, wherein the driving of the fourth gear comprises: coupling a ring gear to a sun gear of the planetary gear train; and driving the ring gear with the third gear.
 19. The method of claim 18, wherein the driving of the ring gear comprises: driving a fifth gear on an input end of a second shaft with the third gear; and driving the sun gear with a fifth gear on an output end of the second shaft.
 20. A method for driving a plurality of engine accessories, each of which is operable at a different speed and/or load than the speed and/or load of the other engine accessories, the method comprising driving the accessories with a planetary gearbox such that gear backlash between the engine and the accessories is substantially confined to backlash between the engine and the gearbox and substantially eliminated between the gearbox and the engine accessories. 